This invention relates to a method for the manufacture of membranes and membrane separation units and the detection and location of leaks. These membranes and membrane separation units are useful in the separation of components from mixtures containing them, especially the separation of olefinic compounds from saturated materials.
It is often desirable to separate components from mixtures for a variety of purposes such as purification, isolation of certain components, and the like. This is often true regarding mixtures containing olefins. For example, Kaplan, U.S. Ser. No. 919,861 filed June 28, 1978 now U.S. Pat. No. 4,154,770 directed to the removal of olefinic compounds from hydrocarbon mixtures containing olefins and saturates so that an olefin rich stream can be passed to an alkylation unit, thereby improving the alkylation process. Also, U.S. Ser. No. 948,339, filed concurrently with this application is directed to recovering unreacted olefinic compounds from polymerization reactor streams so that the olefinic compound can be recovered and recycled. There are a myriad of other instances where various separations are useful.
A most common technique of separating materials is by distillation. However, where the materials have similar boiling points, such as an olefinic compound and its corresponding saturated compound, suitable separation by distillation is expensive. Another method of separating olefinic compounds from saturated materials is through the use of semipermeable membrane separators. The mixture of compounds to be separated contacts one side of a semipermeable membrane and an enriched olefinic stream is removed from the other side. While polymeric membranes can be made which are capable of such separation, the preferred separators use an aqueous liquid barrier containing metal containing ions in conjunction with semipermeable membranes. The metal containing ions are capable of forming reversible water soluble complexes with olefinic compounds. By maintaining an olefinic compound partial pressure differential across the membrane, olefinic compounds are selectively transported across the membrane. This type of separator and process is more fully described in U.S. Pat. Nos. 3,758,603; 3,758,605; 3,770,842; 3,812,651; 3,844,735; 3,864,418; 4,014,665; and 4,060,566 which are hereby incorporated by reference and made a part hereof.
The membrane separator or separation zone comprises a semipermeable membrane and is capable of separating olefinic compounds from mixtures containing them. Preferably the separator comprises a semipermeable membrane which is used in conjunction with a liquid barrier comprising aqueous metal containing ions capable of forming soluble-reversible complexes with the olefinic compound to be separated. By maintaining a suitable partial pressure differential of the olefinic compound to be separated across the membrane, the olefinic compound is selectively transported across the membrane and the liquid barrier so as to effectively and substantially separate the olefinic compound from saturated hydrocarbons. The partial pressure differential across the membrane can be maintained by removing olefinic compound which is passed through the semipermeable membrane with a gaseous or liquid sweep comprising hydrocarbon or nonhydrocarbon compounds, use of vacuum, or other techniques. It is generally preferred to use saturated hydrocarbon sweeps.
The essentially solid, water-insoluble, semi-permeable membranes or films employed in the process of the present invention can be hydrophobic, but preferably are hydrophilic. Hydrophilic membranes permit the liquid barrier to be within the membrane at least to a significant extent. The hydrophilic membranes include membranes which contain additional hydrophilic and/or hygroscopic agents, and those that do not. A film membrane is considered hydrophilic if it absorbs at least about 5 weight percent of water when immersed in distilled water for one day at room temperature and pressure. Typical membranes are those formed of film-forming materials such as nylon, e.g. the N-alkoxyalkyl polyamides, and those formed of nylon and more hydrophilic polymers such as polyvinyl alcohol, polyvinyl ethers, polyacrylamides and the like. A preferred material is cellulose triacetate. The polymer materials can be formed into single membrane structures of desired configurations, as for example, by casting, or they can be formed into hollow fiber films by extrusion from solution or pseudo-solutions and subsequently bundled into an array. The hollow fiber membranes are preferred because they provide a large surface contact area for a given equipment volume. For instance, separation can be achieved using hollow fiber membranes when the feed gas or liquid is passed to the outside of the fibers, the sweep fluid is passed through the inside of the fibers and the material undergoing separation passes from the outside to the inside of the hollow fibers. But preferably the feed is passed through the inside of the fibers, sweep fluid is passed on the outside of the fibers, and the material undergoing separation passes from the inside to the outside of the hollow fibers.
The hollow fibers are commonly anisotropic, having a thin outer skin over a somewhat more porous supporting structure. These fibers are generally kept wet or damp with water because drying can adversely affect the membrane structure. Such fibers are available commercially for use in water desalination. Such separation units have U-shaped hollow fibers held within a container wherein feed is passed into the container and product water is passed through the fiber wall to the fiber bore. The purified water then trickles out a fiber end and out of the separator. This exemplifies a single ended permeator wherein the opposite fiber ends are in communication with essentially the same composition. For water desalination, communication into one end and out the other end of a hollow fiber is not necessary as it generally is in the facilitated transport of olefins. For the latter process, it is desirable to have a double ended permeator so that a feed can be introduced to the membrane and saturate rich effluent removed, while a sweep fluid passes the opposite side of the membranes to remove permeated olefin. A double ended permeator is a permeation unit wherein the opposite ends of elongated hollow fiber membranes are in communication with different compositions. Therefore, it is useful to be able to convert the single ended type permeation unit to a double ended unit.
Hollow fiber membranes are commonly held in place with a potting agent such as epoxy. It has been found that when anisotropic fibers which are potted by certain techniques are used in certain membrane separation units, especially those for the separation of olefinic compounds by facilitated transport, the potted fiber ends are deleteriously affected and leaking occurs. In commercial type membrane separating units, the units have high density packing of hollow fibers. When potting the ends of such units in epoxy, heat from the highly exothermic reaction of the epoxy can damage the fibers. The larger the epoxy pot, the more severe the problem because the pot is self heating on the inside and self insulating on the outside.
During the manufacture of membranes or fibers, irregularities, moles or damaged portions can occur causing such membranes or fibers to be incapable of maintaining fluids, especially liquids, on one side of the membrane or fiber. Also, membranes or fibers can be damaged in the manufacture of separation devices through improper handling and the like, thereby causing the membranes or fibers to leak when used in separation devices. Such leaking causes a loss in selectivity and can cause a loss of aqueous liquid separation barriers, such as aqueous silver ion solution. In order to maintain proper separator performance and selectivity, and to prevent loss of liquids, it is desirable to minimize the number of leaking membranes or fibers. Before the leaking members can be removed or plugged, they must be detected and sometimes located. This can be especially difficult in a large bundle of small diameter hollow fiber membranes due to their small size and congestion of fibers.
It is an object of this invention to provide a method of detecting leaks in membranes exhibiting salt rejection, especially hollow fibers.